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Physical Review Applied

Selective Coupling Enhances Harmonic Generation of Whispering-Gallery Modes

Luke S. Trainor, Florian Sedlmeir, Christian Peuntinger, Harald G. L. Schwefel

Published Feb 7, 2018• 20 citations
Publisher

Abstract

Efficient generation of harmonics in nonlinear optical resonators, particularly in whispering-gallery-mode (WGM) resonators, has led to successes such as parametric oscillators, narrow-band single-photon sources, and versatile frequency converters. A subtle problem, though, has strongly limited efficiency and hindered precise out-coupling of the generated light. Implementing their own prior theory and a polarization-selective birefringent coupling prism, the authors demonstrate an improvement of more than an order of magnitude in the amount of out-coupled light. The results are particularly interesting for applications in quantum optics relying on WGMs.

DOI
10.1103/physrevapplied.9.024007

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Efficient generation of harmonics in nonlinear optical resonators, particularly in whispering-gallery-mode (WGM) resonators, has led to successes such as parametric oscillators, narrow-band single-photon sources, and versatile frequency converters. A subtle problem, though, has strongly limited efficiency and hindered precise out-coupling of the generated light. Implementing their own prior theory and a polarization-selective birefringent coupling prism, the authors demonstrate an improvement of more than an order of magnitude in the amount of out-coupled light. The results are particularly interesting for applications in quantum optics relying on WGMs.
Optical whispering gallery modes (WGMs) derive their name from a famous acoustic phenomenon of guiding a wave by a curved boundary observed nearly a century ago.This phenomenon has a rather general nature, equally applicable to sound and all other waves.It enables resonators of unique properties attractive both in science and engineering.Very high quality factors of optical WGM resonators persisting in a wide wavelength range spanning from radio frequencies to ultraviolet light, their small mode volume, and tunable in-and out-coupling make them exceptionally efficient for nonlinear optical applications.Nonlinear optics facilitates interaction of photons with each other and with other physical systems, and is of prime importance in quantum optics.In this paper we review numerous applications of WGM resonators in nonlinear and quantum optics.We outline the current areas of interest, summarize progress, highlight difficulties, and discuss possible future development trends in these areas.
Single-photon sources play a crucial role in quantum information processing. Among the various methods for generating single photons, spontaneous parametric down-conversion (SPDC) is one of the most widely used. Numerous studies have demonstrated the implementation of SPDC sources across different platforms. Among these, crystalline whispering-gallery-mode resonators (WGMRs) represent an intriguing platform for realizing SPDC-based sources. WGMR-based SPDC sources have been shown to be capable of efficiently interacting with alkali atoms due to the narrow and tunable emission bandwidths. Moreover, they operate with relatively low pump power, which is advantageous for large-scale integration and power-constrained applications. In this work, we further demonstrate the versatility of WGMR-based SPDC sources through three key experiments. First, we generate polarization-entangled photon pairs using an interferometric scheme, achieving a Clauser–Horne–Shimony–Holt (CHSH) inequality parameter of 𝑆 =2.54 ±0.06, violating the classical bound by over eight standard deviations. Second, we observe Hong–Ou–Mandel interference with visibilities of 74 ±5% and 76 ±4% for photons from one and two independent WGMRs, respectively, using an in-coupled pump power as low as 50 nW. Finally, we demonstrate photon--ion interaction between a WGMR-generated photon and a ¹⁴⁷Yb⁺ ion with a success probability of 8%. These results highlight the potential of WGMR-based SPDC sources for scalable quantum networks and hybrid quantum systems.
Quantum information is the next frontier in communications. to realize quantum communications, the quantum mechanical properties of today's best communication medium, light, must be harnessed in a scalable and efficient manner. Whispering-gallery mode resonators (WGMRs), a type of optical cavity, have advantages over traditional designs that can enhance processes used in the generation of nonclassical (quantum) states of light. In particular they reduce the power threshold for intensity-dependent nonlinear phenomena. One such process, second harmonic generation, can reduce the shot noise of light below the standard quantum limit. This dissertation explores the theoretical analysis and experimental tests of noise reduction through second harmonic generation in a crystalline whispering-gallery mode resonator. We also observe the generation of another nonlinear optical process, hyper-Raman scattering, at modest optical powers inside a crystalline WGMR. The change in optical properties of vanadium dioxide due to an optically-induced phase transition is also studied as a potential Q-switching material in a WGMR-type cavity.
We demonstrate for the first time natural phase matching for optical frequency doubling in a high-Q whispering-gallery-mode resonator made of lithium niobate. A conversion efficiency of 9% is achieved at 30 microW in-coupled continuous wave pump power. The observed saturation pump power of 3.2 mW is almost 2 orders of magnitude lower than the state-of-the-art value. This suggests an application of our frequency doubler as a source of nonclassical light requiring only a low-power pump, which easily can be quantum noise limited. Our theoretical analysis of the three-wave mixing in a whispering-gallery-mode resonator provides the relative conversion efficiencies for frequency doubling in various modes.